In general, a seatbelt for constraining a passenger during a collision of a vehicle is installed in a seat of the vehicle to prevent the passenger from being injured while being thrust toward the front side to collide with a vehicle body or an object during the collision of the vehicle.
However, when a vehicle collides, the body of a passenger leans to the front side due to inertia, and a webbing of the seatbelt presses the chest of the passenger strongly.
Meanwhile, when a fastening force of a webbing is made strong to firmly fix a passenger, a passenger is constrained from being thrust toward the front side to collide with the body of a vehicle or an object, making it possible to reduce injury to the passenger. However, the webbing may press the chest of the passenger strongly, causing injury to the chest or stomach of the passenger.
Thus, in recent years, a load limiter for limiting a load between a passenger and a webbing so that the load cannot exceed a preset load range has been installed in seatbelts, whereby a pressing force applied to the chest of a passenger due to a strong fastening force of a webbing can be lessened.
However, the currently developed load limiter is configured to limit a load applied between a passenger and a webbing to a high load range in an initial step where the load limiter starts to be operated, and to lower a load range in the following step.
The fact that a load applied to the chest of a passenger in the initial step of the operation of the load limiter is higher than that of the following step means that a degree by which a pressing force applied to the chest of a passenger is weak at the initial step of the operation of the load limiter, i.e. at the beginning when the vehicle collides and an effect of the load limiter is weak at the initial step of the collision of the vehicle. Thus, demand is increasing for development of a technology for loosening a webbing relatively easily by making a load applied between a passenger and the webbing small at an initial step of an operation of a load limiter so that a pressing force applied to the chest of the passenger can be lessened at an initial step of collision of a vehicle, and then increasing a load applied between the passenger and the webbing so that the webbing cannot be easily released before the head of the passenger collides with a structure of the vehicle disposed on the front side of the passenger.